Energy Transfer in Molecular Dyads Comprising Metalloporphyrin and Ruthenium(II) Tris(2,2‘-bipyridyl) Terminals. Competition between Internal Conversion and Energy Transfer in the Upper Excited Singlet State of the Porphyrin

The photophysical properties of a tripartite supermolecule comprising zinc porphyrin and ruthenium(II) tris(2,2‘-bipyridyl) terminals separated by a trans Pt II bis-σ-acetylide fragment bearing tri- n -butylphosphine residues have been recorded in solution. Thus, excitation into the ruthenium(II) tris(2,2‘-bipyridyl) fragment is followed by fast intramolecular energy transfer to the triplet state of the porphyrin with only a minor contribution from competing (spin-forbidden) triplet-to-singlet energy transfer. Deactivation of the first excited singlet state localized on the porphyrin involves singlet-to-triplet energy transfer to populate the triplet state of the ruthenium(II) tris(2,2‘-bipyridyl) complex, which rapidly transfers excitation energy to the triplet state of the porphyrin. There is no experimental evidence in support of intramolecular electron transfer between the terminals, such processes being inhibited by poor thermodynamics and by the barrier imposed by the central Pt II bis-σ-acetylide fragment. Following excitation into the second excited singlet state of the porphyrin moiety, which has an inherent lifetime of ca. 3 ps, ultrafast singlet-to-singlet energy transfer to the ruthenium(II) tris(2,2‘-bipyridyl) complex competes with internal conversion. Rapid intersystem crossing within the excited-state manifold of the ruthenium(II) tris(2,2‘-bipyridyl) fragment is then followed by slower triplet energy transfer to the porphyrin.